I'm constantly looking for new systems to either steal from or at least try out for a one-shot. What systems are currently available that feature GM-less mechanics? Further, do any of these systems provide advice for adopting their mechanics to other systems (or are just easy to adapt because of the nature of the mechanics)?

10 Answers
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Universalis gives everyone pennies to "pay" to make a thing true. The points replenish by doing things the system wants to encourage, such as getting involved in conflicts, win and lose. This basic dynamic can be adapted in a variety of ways.

Fiasco has a fixed number of dice available to roll. They serve as a pacing mechanic, with the story twist happening when they're half used, and when they're all used the game session ends.

Polaris splits GM authority among all the other players when a player's character is in the spotlight. The player opposite is responsible for presenting conflicts tailored to the spotlight player, while the rest are responsible for the impartial referee role, and for throwing in suggestions when the opposition and spotlight players need ideas.

Polaris also uses ritual phrases to negotiate conflicts, with certain phrases only being able to follow other certain phrases. The thematic effect is to give the game a stately, mythic quality. The mechanical effect is that each phrase presents the other player with a constrained set of choices for how to resolve the conflict, but importantly the constraints are built upon their prior choices, giving them high relevance to the player's goals and charging their choices with character-tailored drama.

Archipelago II (which is free to download) dispenses with dice and uses ritual phrases (similarly but differently to Polaris) to trigger twists or alterations in another player's narration. These phrases have "soft" effects that require a response from the narrating player, but don't force them to narrate something particular, so that they retain full authority over the story they're telling about their character. Ritual phrases are fixed words to speak and have defined effects: things like calling for more detail about something, asking for the player to narrate an unspecified obstacle and how they overcome it, or asking them to narrate in a different direction.

Although many of these shared-authority mechanics could be adapted to use in a GMed game, it occurred to me that the easiest way to use such mechanics would be to structure plot-specific sub-systems or minigames:

Fiasco's limited-dice mechanic could be used to structure a tournament, with one opposed roll for each match determining the specific outcome of the match, and the total sum of a player's rolls determining who wins the whole tourney. That would make match and overall tourney winning slightly decoupled: the effect would be that the player whose character rolled consistently well, even if they didn't win quite as many matches as another, could have the highest total and so "win the king's favour" and be declared the tournament winner; or there could be a by-the-matches winner and a "Best of Tourney" award to two different characters.

Similarly, using ritual phrases might be a way to structure, say, a minigame where two character have to make their case before a town's judge. The interconnections of the ritual phrases would make their arguments tactically-meaningful, and could either be the entire resolution mechanic or provide a way of vying for a bonus to the next roll to convince the judge that their side is right.

having mentioned Polaris, I'd add: take a look at Bliss Stage, by the same designer. It's not quite as GMless as Polaris - one player plays an older, in-charge 'mission briefing' character who can set the mission objectives - but it's very group-narrative based, and has several mechanics easily used for a GMless game. In particular, it has player-controlled NPC relationships, player-designed antagonists, and while the missions are 'GM' designed, players can alter objectives on the fly, and final plot resolution is always player-controlled.
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TynamMay 10 '11 at 12:49

Using the Mythic GM Emulator you can run just about anything you would want without needing a GM — some systems work better than others, of course, but with a little flexibility you should be good to go.

The same company also has World vs. Hero which is a two-player, non-GM game, but I don't have any experience with that game (though there is some discussion in the Yahoo Group).

As for adapting the MGME to other systems: it is essentially a way to answer the "what happens next?" question when you just don't know. As this does happen to actual GMs, the Mythic GM Emulator can be used straight-up as a nice backup tool for a GM.
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SevenSidedDie♦Sep 26 '10 at 4:16

Most of these just apportion the various GM responsibilities to different people at different times. In general, you're never going to get rid of the need for someone to do certain "GMish" things at the table, but you don't need to have a single player doing all of them all the time.

Various GMless techniques you can lift, with examples from some of the games listed above:

Hidden information. In traditional games, the GM owns all the hidden or secret information. Annalise lets every player-character own a secret written by another player and redistributed randomly.

Providing opposition. In traditional games, the GM provides all the opposition against the PCs. In Shock:, each player plays a protagonist, and another player plays his or her antagonist.

Having NPCs at all. The GM in traditional games creates and runs non-player characters (NPCs). In Breaking the Ice, Fiasco, and Kagematsu, you don't really need NPCs. All the action is about the PCs and the conflicts they run into, generally.

Running the NPCs. In traditional games, the GM plays all the NPCs and opposition. In The Shab-al-Hiri Roach, a player can appoint another player on the spot to play a specific NPC. In Polaris, certain players are designated to play certain NPCs, but these roles shift positionally (the player on your right, left, or across from you, etc.).

Pacing. In traditional games, the GM is responsible for the pace of the game, cutting from scene to scene, and making sure every PC gets a fair amount of spotlight time. In Annalise, there are rules that make sure every player is involved in every scene; additionally, there are rules and guidelines for advancing the story toward an ending.

I'd take a hard look at Polaris, which has a track record of being adapted (Anna Kreider's game Thou Art But A Warrior uses the core Polaris mechanic but re-skins it in a cool way). Polaris has clearly defined roles that rotate with the players (protagonist, antagonist, and two supporting roles) and a system of conflict resolution that relies on specific phrases.

Funny, I've hacked Archipelago II several times and totally forgot to mention it. It is more extensible and flexible than Polaris and very easy to modify.

I'm currently running a 4E D&D campaign, and one experiment that's worked out very well for me as a DM is using Microscope to build out the history of the world. It is a completely GM-less game, and has worked incredibly well for building up a homebrew campaign settting. However, I think it can pretty painlessly be integrated not only into a DM's toolbox, but into the entire group's play of the game.

Since Microscope is about building out some chunk of world history, you could use it as a springboard for a D&D game. Build a history with a group, then drop them into the world and let them loose - in my experience, Microscope leaves those who play it with a fairly strong interest in the world that's been crafted. It could be integrated into the active play of a GM-based game I imagine, but it doesn't provide explicit suggestions on how to do so.

I would recommend you take a look at Dirty Secrets, a very interesting system dedicated to creating a modern noir. My group has had a couple of fun sessions with Dirty Secrets. Adapting to another genre would take some work, but I think it could be done. There's no GM but there is an "investigator," a role taken by a single person for the whole game. The rest of the players rotate through roles as support and / or opposition through the game. There's a lot to love about Dirty Secrets, including the fact that it knows when it is over.

I have also played, with less success, The Committee for the Exploration of Mysteries, another example of the "ritual phrases" mechanic which nobody in my group liked. They rejected the idea of being told how to play. But that doesn't mean the game wasn't interesting from a mechanical standpoint. I can't provide a link because of spam prevention.

Another game that has not been mentioned is Western City. We used it for a western city as well as our village in a Song of Ice and Fire RPG. I would use it again in such a situation: if you need to generate some NPC personalities and some events in a village to use as backdrop for the "real" game. A nice one or two session GM-less break from the regular game.

I took the liberty of adding a link to Mongoose's product page for Western City. Also, that's a nice-looking and apparently flexible game. I'm keen on games that are GMless and good for both one-shots and ongoing games—I've ordered a copy, so thanks. :)
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SevenSidedDie♦Sep 27 '10 at 17:57

Try taking a look at Wizards' Castle Ravenloft board game. It has been a while since I played it, but each turn a player places a map tile, puts a monster in play or other action from their hand of cards, and moves some (or all, i forget) of the monsters on the board in the direction of an ally. It basically is a DM-less dungeon raid, and does a pretty good job of it. Definitely worth looking into. It is also perilous. Like with a real DM, there is a very real chance the quest will fail and you will all die.

I guess the subject does state rpgs. However, the post content does not, using the more ambiguous term "systems" instead. Castle Ravenloft is very much a DM-less D&D 4th edition. If the OP is looking for complete systems, this wont help, but he specifically said other systems to "steal from or try out as a one shot". Ravenloft is very much worth looking at for ideas to steal. If you think my suggestion is completely without merit, then downvote me. I'm done defending this.
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Arr MiHardiesSep 27 '11 at 4:58

The rotational GMing works by, for each scene, one player takes the additional role of "Lead Narrator". To enable this to work effectively, the rules are kept very simple, limited modifiers exist, and with a large amount of randomness in the rolling. After being lead narrator for one scene, the Lead Narrator duty falls to the next player around the table.

For non-combat tasks, the lead narrator picks the relevant attribute (of 5 choices), and a modifier between +3 and –3. The Narrator also rolls 1d20 to set the target. The acting character's player rolls 1d12+1d(attribute)+ the modifier; roll higher than the d20 to succeed.

The story is comprised of a group of 3 or more linked scenes, each with a narration setup to be read by the new Lead Narrator, and a list of things or beings to be dealt with in that scene. The core book and Into The Cosmos scenes are fairly linear; none have flow-chart style choices, but a new adventure book with flow based upon scene outcome is due out.

Pregenerated characters allow both faster starting in to play, and no need for Character Generation being overseen.

The very campy setting has about 20 pages of setting in a gazeteer section, designed for use on the fly, and it's pretty well vague about much of it. Just enough to really get a sense for how to build more scenarios. That the setting is campy also allows for more colorful play, and this is encouraged by use of "cues" on both scenes and PC sheets. The pregenerated characters (both Player and opponents) also have cues, most of which seem to be taken from the movies and TV shows by either borrowing a line exactly, or changing the enemy name to match the custom setting.

The focus on action scenes allows easier narration, but it is explicit that if the next scene doesn't fit, an additional, improvised scene can be added. Further, the genre, campy as it is, is overrun with short bits of cheesy dialogue to bridge one action scene to another.

"Yes And" as a game rule is both how the game really works, and its biggest drawback as a mechanic. The game works because you're not allowed, even when the lead narrator, to counter narration, except by the LN requiring a roll to succeed. Thus, the story moves on, and arguing about what's been added so far is explicitly not allowed. On the other hand, this also requires the players all agree that it's story time, not "Let's beat the GM's Thugs Time." Players out to "win" by success can narrate some pretty wild and unpleasant things; with no central authority to say "too far," players must exercise some restraint.

Plot points are used in the game. They allow players to add +1 to +3 to any roll (including the Lead Narrator's d20, if desired), add an alien, have an unseen alien attack, change the turn order for combat, recover health or armor, make additional effects besides the expected happen, or trigger plot twists.

Things that can be ported:

Rotational GMing

Simple mechanics allow for better play

Combat heavy settings allow for linear adventures to work better

separate scene sheets for every scene can be used in a manner that enables play with surprises intact.

Yes, And. (Which appears in many games listed in other answers.)

Plot Points

Adaptation

None is given.

The adventures need only relevant opponent stats to work with about any game system. They're simple, straightforward, and colorful.

The Plot Point mechanics are almost divorced from the others; only the modifier limit needs to change to use this with other games.

"Yes, And" is already in many games, and can easily be ported into other games that lack that explicit advice.

Rotational GMing can be ported because it's procedural, rather than explicitly mechanical

The general action mechanics can be ported as a whole, but being as simple as they are, may be unsatisfying.

The game as a whole can also readily morph into a GM-run game with no shared GMing; the setting is just enough to allow this with few difficulties.

I can suggest a look on Ars magica. The Storykeeper system favors:
Each players creates and can play more than one char at same time;
GM rotation for each history arch, GM also gets some PCs;
Also players can swap GM position for different "assemblies" of PCs
It can be used to allow parallel archs or/and different GMs working each week.
Favor scenario over system;